Lubricating fluid for an axle assembly is generally expected to prevent metal to metal contact between the relatively moving components. The fluid also acts as a coolant by absorbing the heat generated from friction and torque from the moving parts of the assembly. Axle assemblies are generally lubricated by a splash system. The outer housing of the differential assembly contains a reservoir of lubricating fluid. Part of the ring gear of the differential is immersed in the fluid. As the ring gear rotates, fluid splashes from the reservoir upward onto the other rotating parts of the differential and the axle assembly to lubricate them, and eventually drains back to the reservoir.
The lubricating fluid acts to remove heat from the components of the axle assembly, so the splashed fluid returns to the reservoir at an elevated temperature. For optimal performance, the fluid in the reservoir should be cooled. This can be accomplished with a cooler located external to the axle assembly. The heat from the lubricating fluid can be passed to the ambient air by convection in the external cooler. A pump is required to circulate the lubricating fluid between the reservoir and the cooler.
An example of such a pump is disclosed in U.S. Pat. No. 6,189,411 to Francis (“the '411 patent”), which is hereby fully incorporated by reference. The '411 patent discloses an axle assembly 100 that includes a gear pump for circulating fluid between a reservoir and an external cooler. This axle assembly can be seen in FIGS. 1-3. Referring to FIG. 1, the pump 101 is located inside the axle housing 102. Referring to FIG. 2, the pump assembly includes two intermeshing gears 103 and 104 that rotate to pump lubricating fluid from an inlet port 105 connected to the fluid reservoir at the bottom of the axle assembly to an outlet port 106 connected to the external fluid cooler (not shown). Referring to FIG. 3, gear 103 of the gear pump is directly connected to the end of the pinion shaft 107. The gear 103 is mounted on a hex spline 108 that fits within a hex bore 109 in the pinion shaft. The pinion shaft 107 is also geared to the ring gear 110 of the differential, which can be seen in FIG. 1. In particular a pinion gear 112 that is mounted on pinion shaft 107 meshes with the ring gear 110. The pinion shaft 107 rotates at the same speed as the output shaft of the transmission and is connected to the transmission output shaft by, for example, a drive shaft and two universal joints.
The '411 patent describes the axle assembly disclosed therein as being suitable for the rear axle of a racing vehicle. This is because the positioning of the gear pump inside the housing of the axle permits the ring gear of the differential to be replaced without removing the gear pump. (In racing vehicles, the ring gear of the differential is frequently changed, especially just before a race, to fine tune the gear ratio of the rear axle assembly.) It has been found, though, that in practice such an axle assembly, in particular the cooling system for the lubricating fluid, is not sufficient for race vehicles, for example a NASCAR Busch series vehicle. It has been found that the lubricant in the reservoir remains at too high of a temperature for too long and that the temperature of the lubricant fluctuates within too large a range.
What is needed in the art is an axle assembly that can maintain the lubrication fluid in the reservoir at a lower temperature and that can reduce the range of fluctuation of the temperature of the lubrication fluid in the reservoir.